Automobile power steering systems include the one wherein a concave or convex supported faces are provided on the back side of the rack teeth of a rack bar, and by pushing down on the supported faces with a yoke to support the back side of the rack bar, a pinion gear and the rack are made to be in well-engaged state (Patent Literature 1).
A solid rack bar is used for ordinary power steering systems. By forging a solid material, a rack portion and V-shaped or Y-shaped supported faces are formed, the supported faces are provided on the back side of the teeth of the rack portion (Patent Literature 2).
In the case of solid rack bar, since it is difficult to manufacture a variable rack by machining, wherein the rack tooth pitch and the shape change along the axial direction, the variable rack is made by forging.
Meanwhile, it is known that the weight of a power steering system can be reduced much substantially by using a hollow rack bar as shown in Patent Literature 3. In Patent Literature 3, by placing a toothed mold on the outer face of a circular tube material, and by inserting a cored bar to allow the inner face of the material to be under high pressure, the shape of the toothed mold can be transferred to the outer face to form the rack teeth with high precision.